Many conventional types of photoconductive (PC) mercury-cadmium-telluride (HgCdTe), and other low impedance detectors, require separate biasing circuitry for each detector element of an array of such detectors. This requirement makes the layout and subsequent fabrication of large area PC arrays difficult to accomplish. This requirement also complicates the provision of support circuitry and cabling, and also increases the power consumption of the cabling.
In general, each element of the array requires two current carrying connections or leads to provide an input and an output for a bias current. During operation, the array is positioned at a focal plane of a suitable optical system. The absorption of infrared (IR) radiation by a photodetector element causes a change in the resistance of the element. In a constant current mode of operation, this change in resistance can be sensed as a change in potential across the element. Signal processing circuitry, including amplifiers and multiplexers, measures and reads out the change in potential, which is an indication of the magnitude of the IR flux that is incident on the element.
The array may be provided in a front-side illuminated configuration, wherein the elements are fabricated upon a radiation receiving surface of a suitable substrate. In the front-side illuminated configuration, the required signal processing circuitry may be provided at a periphery of the substrate or upon a back-side of the substrate. In a back-side illuminated configuration, the array is upon a surface of a transparent (at the wavelengths of interest) substrate, and the radiation is incident upon the opposite surface of the substrate. The radiation passes through the substrate to the array elements. For the back-side illuminated configuration, conventional practice often places the signal processing circuitry upon a separate substrate which is mated, or hybridized, with the radiation detector array substrate. Interconnects between the two substrates may take the form of indium bumps. The bumps are formed at required locations on the radiation detector and are subsequently mated with appropriately placed contact pads on the signal processing circuit substrate.
The aforementioned problem of providing two current carrying terminals to each element of the array is present in both the front-side illuminated and the back-side illuminated configurations.
It is thus an object of this invention to provide an improved array of photoconductive elements that does not require a separate pair of current carrying contacts for each element of the array, thereby overcoming the restrictions present in the prior art.
It is further object of this invention to provide only two current carrying leads for a plurality of photoconductive elements, thereby facilitating the fabrication of on-focal plane integrated circuits for amplification and multiplexing, and for also providing a significant reduction in support electronics, cabling, and power consumption.